1. Field of the Invention
The present invention relates to an electroacoustic transducer for use in a sounder and the like for converting electric signals into sound through electromagnetic conversion.
2. Description of the Related Art
FIGS. 13 and 14 illustrate a conventional electroacoustic transducer. The electroacoustic transducer is provided with a casing 100 formed of a synthetic resin, inside which a holder member 102 made of a non-magnetic material or the equivalent is securely held, and a diaphragm 104 prepared from a magnetic material in a plate form is installed on the upper surface of the holder member 102. A magnetic piece 106 is attached to the diaphragm 104. A resonance chamber 108 is formed on the upper side of the diaphragm 104 with the casing 100 surrounding it, and exposed to the air via a sound emitting hole opened on the casing 100. On the back side of the diaphragm 104, a base 112 and a printed circuit board 114 are installed, closing the back of the casing 100. A magnetic core 116 is installed at the center of the base 112, and wound around with a coil 118. Further a magnet 120 is installed around and spaced apart from the coil 118. An air gap 122 is provided between the top end of the magnetic core 116 and the diaphragm 104, and terminals of the coil 118 wound around the magnetic core 116 are connected to base parts of terminal pins 124 and 126, respectively by means of soldering. The terminal pins 124 and 126 are securely attached to the printed circuit board 114 by clamping the base parts thereof.
Meanwhile, since the electroacoustic transducer described is electrically connected by means of the reflow soldering process to printed circuit boards of various electronic equipment requiring emission of sound, it is heated up at the time of applying the soldering process. Accordingly, a countermeasure for enhancing heat-resistance of relevant components, and preventing degradation in acoustic performance of the equipment is implemented in order to protect the electroacoustic transducer from thermal degradation due to such a heat treatment. Use of heat-resistant components, however, create a cause for an increase in manufacturing costs of the electroacoustic transducer.
A problem with the countermeasure for enhancing heat resistance of the electroacoustic transducer is how to ensure heat resistance of the magnet 120 and the holder member 102, affecting most the characteristic of a magnetic circuit. In particular, heat resistance of the holder member 102 is essential to keep the air gap 122 constant because thermal deformation of the holder member 102 has an effect on a spread of the air gap 122 between the diaphragm 104 and the magnetic core 116.
The magnet 120 undergoes reversible demagnetization at a temperature on the order of 80.degree. C., which is an operating temperature for a sounder, but does not undergo irreversible demagnetization. Therefore, it can restore magnetic force at room temperature. Temperatures at which the reflow soldering is applied are high ranging from 200 to 250.degree. C., and when the magnet 120 is subjected to such high temperatures, irreversible demagnetization occurs, resulting in reduction of magnetic force by about 5 to 30% after it is cooled down to room temperature. It is well known that a degree of such demagnetization varies widely depending on a grade of a material forming the magnet 120, and in general, the smaller a degree of demagnetization is, the higher the cost of a material having such property. That is, the cost of a material having excellent heat resistance tends to rise correspondingly.
FIG. 15 illustrates a profile of the reflow soldering temperatures by way of example. In this case, temperatures measured indicate those at the center of a standard printed circuit board of an electronic equipment. It is shown clearly from the profile that the electroacoustic transducer mounted on the board is subjected to substantial heating, and hence, demagnetization of the magnet 120 is not negligible.
Such demagnetization causes magnetic attraction between the diaphragm 104 and the magnetic core 116 to be decreased, affecting acoustic performance of the electroacoustic transducer. FIG. 16 shows frequency characteristic (acoustic characteristic) of sound pressure prior to the reflow soldering, FIG. 17 frequency characteristic (acoustic characteristic) of sound pressure after the reflow soldering, FIG. 18 frequency characteristic (acoustic characteristic) of electric current prior to the reflow soldering, and FIG. 19 frequency characteristic (acoustic characteristic) of electric current after the reflow soldering. That is, in the case of the electroacoustic transducer subjected to heating during the reflow soldering, a minimum resonance frequency Fo of the diaphragm 104 is decreased, and sound pressure levels also become lower, degrading the acoustic characteristic thereof.
The holder member 102 is formed of a non-magnetic metal, resin, or the like. The holder member 102 formed of such materials undergoes elongation at a temperature in the order of 80.degree. C. according to the linear expansion coefficient of the respective materials, however, restores its initial dimensions at room temperature. By applying temperatures for the reflow soldering, however, the holder member 102 formed of a resin undergoes dimensional contraction due to annealing effect or thermal degradation. FIG. 20 shows contraction ratios of LCP materials. A degree of such contraction varies widely depending on the grade of the respective matierals, and in general, the lower a contraction ratio is, the higher the cost of a matieral having such property.
Contraction of the holder member 102 due to heating during the reflow soldering causes the air gap 122 between the magnetic core 116 and the diaphragm 104 to be narrowed to an extent of the contraction, and magentic attraction bdtween the diaphragm 104 and the magnetic core 116 is strengthened accordingly. Table 1 shows the relationship between materials compsoing the holder member and variation in the air gap spread, and Fig. 21 shows variation in the ari gap spread following the reflow soldering.
TABLE 1 ______________________________________ Results of Survey on Variation in the Air Gap before and after Reflow Soldering after molding reflow air gap model material initial soldering variation (unit: .mu.m) ______________________________________ A VECTRA 191 191 0 " 192 192 0 " 193 193 0 " 191 191 0 AVE. 0.0 " 188 188 0 .delta.n - 1 -- " XREC 189 186 -3 " 188 187 -1 " 192 192 0 " 187 184 -3 AVE. -2.0 " 186 183 -3 .delta.n - 1 1.41 " AMODEL 190 188 -2 " 190 189 -1 " 195 193 -2 " 189 186 -3 AVE. -2.2 " 193 190 -3 .delta.n - 1 0.84 B VECTRA 163 160 -3 " 155 150 -5 " 133 114 -19 " 152 154 2 AVE. -5.8 " 164 160 -4 .delta.n - 1 7.85 ______________________________________
In order to avoid adverse effects as described above of the reflow soldering temperatures, a magnet having a low ratio of irreversible demagnetization has been used for the magnet 120, and a resin, metal or the equivalent having a low contraction ratio has been used for the holder member 102. Such a practice, however, has created a problem of an increase in the costs of components of the electroacoustic transducer.